The natural sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
To facilitate direct stimulation of the auditory nerve fibers, an electrode array portion of a lead may be implanted in the cochlea. Electrodes included on the electrode array portion form stimulation channels through which electrical stimulation pulses may be applied directly to auditory nerves within the cochlea. An audio signal may therefore be presented to a patient by translating the audio signal into electrical stimulation pulses and applying the stimulation pulses directly to auditory nerves within the cochlea via one or more of the electrodes.
The electrode array portion is often implanted within the scala tympani, one of three parallel ducts that make up the spiral-shaped cochlea. Electrode array portions that are implanted in the scala tympani typically include several separately connected stimulating electrodes (or “electrode contacts”) longitudinally disposed on a thin, elongate, and flexible carrier. Such an electrode array portion is pushed into the scala tympani duct via a surgical opening made in the cochlea wall at or near the round window at the basal end of the duct.
During use, electrical current is passed into the fluids and tissues immediately surrounding the individual electrical contacts in order to create transient potential gradients that, if sufficiently strong, cause the nearby auditory nerve fibers to generate action potentials. The auditory nerve fibers arise from cell bodies located in the spiral ganglion, which lies in the bone, or modiolus, adjacent to the scala tympani on the inside wall of its spiral course. Because the density of electrical current flowing through volume conductors such as tissues and fluids tends to be highest near the electrode contact that is the source of such current, stimulation at one electrode contact site tends to selectively activate those spiral ganglion cells and their auditory nerve fibers that are closest to that contact site.
Hence, it is often desirable for the electrode contacts to be positioned as close to the ganglion cells as possible and/or to any other location (e.g., a mid-scalar location) as may serve a particular application. To this end, various electrode array portions have been developed that have resilient carriers configured to better conform to the shape of the scala tympani and/or other auditory structures.
Unfortunately, many conventional insertion tools used to insert electrode array portions into the cochlea are cumbersome and difficult to use. For example, it is often difficult to release an electrode array portion from an insertion tool once the electrode array portion has been inserted into the cochlea. In addition, a stiffening member (e.g., a stylet) may be used to facilitate insertion of the electrode array portion of a lead into the cochlea, and retracting the stiffening member from the electrode array portion may be difficult and tend to dislodge the electrode array portion out of position.